This invention broadly relates to television reception and, more particularly, to a novel method and circuit for distortion-free detection of the intercarrier sound signal by mixing the sound intermediate frequency signal with a pure unmodulated signal which is derived from the oscillator output of the video synchronous detector and, accordingly, phase-locked to the video intermediate frequency signal. While the invention is generally useful in all intercarrier television receivers, it has particular utility in subscription television receivers where distortion or loss of the sound signal cannot be tolerated because of audio encoding techniques involving modulation of non-standard frequencies as well as juxtaposition of sound frequencies within the broadcast band.
In known monochromatic receivers, the respective sound and video intermediate frequency signals are amplified in the video intermediate frequency amplifier and then heterodyned in the video detector or in the intercarrier sound detector to produce the intercarrier sound signal. In known color receivers, the sound intermediate frequency carrier must be severely attenuated in the video intermediate frequency amplifier to prevent the interference pattern which would be caused if the sound intermediate frequency carrier and color subcarrier were allowed to beat together in the video detector. Accordingly, in such receivers, sound and video intermediate frequency signals are obtained from a point preceding the video detector and supplied to a separate sound detector for producing the intercarrier signal. The output of the video intermediate frequency amplifier is usually selected as the take-off point for the intermediate frequency signals.
Although differing in point of signal take-off and means of detection, recovery of the sound intercarrier in both of the above known receivers is effected by mixing the respective video and sound intermediate frequency signals. Since this mixing is governed by a multiplication process between the respective amplitude and frequency components of the wave forms representing the video and sound intermediate frequency carriers, production of the intercarrier sound signal, and, hence, recovery of the transmitted sound information, is dependent upon the video content, i.e. amplitude, of the video carrier. Thus, receivers employing this intercarrier detection technique suffer from the disadvantage that distortion of the video carrier results, as well, in unavoidable distortion of the recovered sound. Such distortion is typically occasioned by over-modulation of the video carrier beyond the prescribed maximum modulation depth, as for example, when a local affiliate superimposes call letters or messages over a network broadcast. Even in the absence of over-modulation, other signal degradations affecting the video carrier, such as noise, and preshoots and overshoots generated by envelope delayed distortion, can likewise produce sound distortion in receivers using known intercarrier recovery techniques.